36 PAUL PELSENEER. 



B. Lamellibranchia Protobranchia (Nucula, &c.) : the 

 genital gland opens at the junction of the coelom (pericardium) 

 and the kidney. 



ii. If we consider now the two classes where only the 

 dioecious condition exists, we find that they have preserved 

 many primitive traits, especially in the reproductive apparatus : 

 the genital gland opens into the coelom (Cephalopoda) or into 

 the kidney (Scaphopoda) . 



These facts show that in the Mollusca the dioecious con- 

 dition has preceded the monoecious, and by no means (as 

 Rouzand has tried to make out, 48) that " les Gastropodes 

 unisexues se presentent reellement comme les descendants des 

 Gastropodes hermaphrodites, plus ou moins analogues ^ ceux 

 qui vivent encore de nos jours." 



The molluscs which are certainly the most archaic are 

 dioecious. Their genital organs present a conformation analo- 

 gous to the primitive disposition observed in the ontogeny 

 of other forms, which can on no account be regarded as the 

 last term of a retrogressive evolution — evolution not being 

 reversible. 



But hermaphroditism has been able to establish itself, re- 

 placing the dioecious state in Mollusca of different grades of 

 specialisation — 



A. In forms where the genital glands open into the coelom 

 (Neomeniidse) ; 



B. In forms where these glands open into the kidneys 

 (Pecten) ; 



c. In forms where these glands have special ducts, ac- 

 cessory glands, copulatory organs, &c. (Onchidiopsis, 

 Euthyneura, &c.). 



2. Hermaphroditism has been established upon the 

 female organism. — The hermaphrodite condition, then, is 

 secondary in Mollusca. As for the organisation which has 

 given rise to it, I believe it to be that of the female. 



Comparative study of the genital ducts and of their develop- 

 ment lends support to this opinion, as we shall at once see. 



i. In Lamellibranchs, when there is a separate male and 



